A first distinction: envelope, pockets, wallet

Pockets usually open at the narrowest side, the back of the wallet is formed by the side seams.

Photographic wallets are generally rectangular, usually with a closing flap longer than the wallet which is not sealed as they are not circulated by the post.

ENVELOPES:
Formed by the bottom flap

POCKETS:
Formed by one or two of the side seams

PHOTOGRAPHIC WALLET:
Formed by the bottom flap

Types of Envelopes

The Spanish norm defines the envelope as "a flat, rectangular-shaped wrapper, generally obtained from a sheet of paper folded in such a way that a single front side is obtained and a back side formed by four flaps which are mounted together.